An apparatus called a NAS (Network Attached Storage) or a file server is generally used in order to store files for multiple client computers (hereinafter simply referred to as the “clients”). The file server stores files created by the clients by reading data from or writing data to a storage system which can be accessed in units of blocks.
Furthermore, recently, while the necessity to store a considerable amount of files is growing, there is a demand for the reduction of a unit price per storage capacity. Patent Literature 1 discloses a technology of migrating files that are stored in a high-speed storage apparatus and have not been accessed for a certain amount of time, to a low-speed storage apparatus without changing a method of access from clients where the high-speed storage apparatus and the low-speed storage apparatus are coupled to a controller capable of processing file-level read and write (which corresponds to the above-mentioned file server). This technology is hereinafter referred to as the “file level hierarchical storage management technology.”